/*---------------------------------------------------------------------------------------------------
*       IMU message structure.  Information sent by ISC control computer 8 times per second.                             

*       Periodic Interval Timer (PIT) is ultimately is derived from the 32.768 KHz CPU oscillator.
*       this oscillator produces a 16,777,000 CPU clock divided by 16 = 1,048,562 Hz.

*   
*    1 PIT interrupt  = 1/1024 sec = .9765625 msec (PIT_count)
*   16 PIT interrupts = 1/64   sec = 15.625   msec
*  128 PIT interrupts = 1/8th  sec = 125      msec (exactly!)   This is the IMUMSG reporting interval.

*---------------------------------------------------------------------------------------------------*/
/* ISC to NAV 8 times per second   */
struct IMUMSG {                     /* Synchronization word                                 */
        USHORT unique_word;         /* Word 1, 0xff80 - 0xff first, 0x81 second             */
        USHORT msgID;               /* Word 2,                                              */
        USHORT msglength;           /* Word 3, Number of words in the structure.
                                               Including unique_word and checksum           */

        short   body_roll_rate;     /* Word 4, Current instantaneous sample. Integer Units = .01 degrees/sec */
        short   body_pitch_rate;    /* Word 5, Current instantaneous sample. Integer Units = .01 degrees/sec */
        short   body_yaw_rate;      /* Word 6, Current instantaneous sample. Integer Units = .01 degrees/sec */

        short   body_lift_accel;    /* Word 7, Current instantaneous sample. Integer Units = .01 meters/sec/sec */
        short   body_thrust_accel;  /* Word 8, Current instantaneous sample. Integer Units = .01 meters/sec/sec */
        short   body_side_accel;    /* Word 9, Current instantaneous sample. Integer Units = .01 meters/sec/sec */

        short   tilt;               /* Word 10, Local horizontal to Body frame side. Integer Units = .01 degrees */
        short   ascension;          /* Word 11, Local horizontal to Body frame nose. Integer Units = .01 degrees */
        short   heading;            /* Word 12, Body frame nose to North. Integer Units = .01 degrees
                                                North = 0, East = , West = -90, South =  180     */

        ULONG   PIT_count;          /* Periodic Interval Timer interrupt count at sample time.
                                       1/8th second sample seperation time. (128 PIT interrupts) */
        

        USHORT  checksum;           // Word 14.     The method is to sum the entire structure from the unique_word
                                    // up to, but excluding the checksum word.  All math is performed using
                                    // signed 16 bit integers with carries, or overflows, ignored.  The sum is 
                                    // negated.
};
typedef struct IMUMSG IMUMSGst;